Antifogging plastic lens material

ABSTRACT

An antifogging plastic material is disclosed that is particularly suitable for use as a disposable fog-resistant lens. The lens material comprises a substantially transparent polyester substrate; a layer of polyvinylidene chloride-containing polymer on the polyester substrate; a water permeable colloid layer on the polyvinylidene chloride-containing layer; and a polyvinyl alcohol-containing layer on the water permeable colloid layer.

FIELD OF THE INVENTION

The present invention relates to antifogging techniques and inparticular relates to a method of coating polyester film to give itantifogging properties, and a resulting antifogging lens material.

BACKGROUND OF THE INVENTION

Protective face coverings, including lens-containing structures such asmasks and goggles that cover the eyes while still providing appropriatevisual acuity, are used in a number of applications. These include eyeprotection in manufacturing or industrial circumstances, laboratories,educational activities, home and garden tasks, and in medicalapplications such as surgery.

With respect to medical applications, the eyes provide one of the mainavailable entry points into the human body for viruses, microbes andother organisms. Thus, in medical practices such as surgery, whereexposure to blood and other bodily fluids of a patient are expected, eyeprotection represents more than protection against loss of vision, andspecifically provides a barrier against entry of potentially serious oreven fatal diseases. For example, the increase in Acquired ImmuneDeficiency Syndrome (AIDS) in the population as a whole, and the presentlack of a known cure, has increased the risk that infected patients ortheir blood will transmit the virus to exposed medical personnel throughtheir eyes.

From a medical perspective, one desirable characteristic of anappropriate face mask, goggle, or other lens is that it be easily usedin a sanitary condition. The simplest method of maintaining a goggle ora mask in a sanitary condition is to manufacture it under sterileconditions, use it once and then dispose of it. Accordingly, desirablemasks and goggles should be manufactured from materials for which thecost is low enough to make disposal practical. One such material ispolyester. When properly manufactured using known techniques andequipment, polyester is transparent, rigid enough to form an appropriateshape, optically clear, and flexible enough to be formed into lightweight, comfortable masks, goggles and similar lens structures.Furthermore, as evidenced by its wide use in disposal food packaging,polyester is a relatively low cost material. It can also be convenientlyrecycled. Thus, goggles or masks formed from polyester can provideappropriate protection at low enough cost to be considered disposable.

One problem that arises with lens-containing devices, however, is theproblem of fogging. In general, fogging occurs on a surface when thetemperature of that surface falls below the dew point of the surroundingambient air. With respect to lenses, fogging tends to occur because ofthe proximity of the surface of the lens portion of a goggle or a maskto the head and eyes of the wearer. As the wearer perspires or warms theair between the eyes and the lens (and even if not visibly so), moisturepresent between the lens and the wearer has a tendency to condense onthe interior surface of the lens. If the lens material is hydrophobic,the condensed moisture will fail to wet the surface, and instead willform small droplets. Observed cumulatively, these droplets diffractlight sufficiently to interfere with or even block the vision of thewearer. Thus, fogging is most desirably avoided where delicate workrequires high visual acuity, such as during medical procedures, some ofwhich are carried out at relatively low temperatures for medicalreasons. For example, some surgical procedures are carried out atambient temperatures as low as 50° F. in order to help slow a patient'smetabolism.

Polyester, although favorable in a number of qualities discussed above,is nevertheless one such hydrophobic material and is thus subject tofogging. Accordingly, in order for polyester to be useful as a lensmaterial in medical and other such applications, some technique must beused to minimize or eliminate fogging.

A typical method of attempting to avoid or eliminate fogging is to addsome sort of hydrophilic coating to a hydrophobic substrate. Whenmoisture condenses on a hydrophilic surface, it tends to wet the surfacerather than form drops. As a result, the wetted surface is oftentransparent enough to prevent vision through the lens from beingimpaired.

In other techniques hydrophobic antifogging coatings have been appliedto lenses in an attempt to cause water vapor to avoid contact entirelywith the lens. Unfortunately, such techniques tend to have a net effectof instead allowing large water droplets to form, which is asundesirable as the fogging.

Even the hydrophilic coatings, however, include certain problems.Typically many exhibit poor optical clarity, cracking, streaking orhaziness, opalescence, adhere poorly to substrates, have an oilysurface, and are difficult to apply in a uniform coat.

A number of other techniques include the use of silicon compounds asantifogging agents, but such techniques bring their own set of problemsare relatively expensive for a disposable article. Other compounds areinsoluble in appropriate coating solvents and thus raise applicationdifficulties.

One potentially favorable hydrophilic coating is polyvinyl alcohol(PVOH). As known to those familiar with this compound, polyvinyl alcoholis produced by the polymerization of vinyl acetate, followed byhydrolysis to form alcohol functional groups.

Polyvinyl alcohol, however, generally will not adhere to common lenssubstrate materials such as polyester, polycarbonate, acrylic, orcellulosic films. This lack of adhesion has limited the use of polyvinylalcohol and the advantages it otherwise offers.

Accordingly, many modifications have been attempted in order to increasethe affinity of polyvinyl alcohol for a substrate such as polyester. Forexample, some techniques add other functional groups to the backbone ofthe polymer in an effort to increase the affinity to the substrate. Asthe chemical nature of the polyvinyl alcohol is modified, however, thehydrophilic nature is also modified and its effectiveness as anantifogging material is minimized or destroyed.

Other techniques attempt to add wetting or surface active agents to thepolyvinyl alcohol, but these materials are only effective when theybloom to the surface of the polyester and in doing so exhibit an oilycondition which in turn creates optical problems. Typically a lensmaterial containing such surfactants creates a mottled appearance whichis generally unacceptable.

Other methods of preparing the substrate have similarly failed toprovide desired results, with such techniques including corona dischargetreatment of the substrate, or of the coatings and primers. Suchtechniques have generally lacked success, however, usually because ofpoor adhesion to the substrate, poor optical properties or both.

OBJECT AND SUMMARY OF THE INVENTION

Therefore, there exists the need for a lens material for protectivegoggles, masks or glasses that can be formed from an inexpensivematerial such as polyester, and that can be coated with an inexpensiveantifogging material such as polyvinyl alcohol and yet while exhibitingthe necessary adhesion between the polyvinyl alcohol and the polyesterwhile maintaining the required visual acuity and physical stability foruse in demanding circumstances.

The present invention meets this object by providing an antifoggingplastic film structure formed of a polyester substrate, a layer of apolyvinylidene chloride-containing polymer on the polyester substrate, awater permeable colloid layer on the polyvinylidene chloride-containinglayer, and a polyvinyl alcohol-containing layer on the water permeablecolloid layer.

The foregoing and other objects, advantages and features of theinvention, and the manner in which the same are accomplished, willbecome more readily apparent upon consideration of the followingdetailed description of the invention taken in conjunction with theaccompanying drawing, which illustrates a preferred and exemplaryembodiment, and wherein:

BRIEF DESCRIPTION OF THE DRAWING

The FIG. 1 is a cross-sectional view of the structure of an antifoggingpolyester according to the present invention.

DETAILED DESCRIPTION

The present invention is an antifogging plastic lens material that isparticularly suitable for use as a disposable fog-resistant lens. FIG. 1illustrates the invention and in which a lens structure is broadlydesignated at 10. The lens material is formed of a substantiallytransparent polyester substrate 11, a layer of polyvinylidenechloride-containing polymer 12 on the polyester substrate 11, a waterpermeable colloid layer 13 on the polyvinylidene chloride-containinglayer 12 and a polyvinyl alcohol-containing layer 14 on the waterpermeable colloid layer.

In preferred embodiments, such as use for lenses in disposal medicalgoggles or masks, the polyester substrate is between about 4 and 7 milsthick (1 mil=0.001 inch). In commercial embodiments, such polyestersubstrate material is sometimes sold as a "nominal" 4 mil or 7 milthickness which may be slightly different from an exact 4 or 7 milthickness.

The polyvinylidene chloride-containing layer 12 preferably contains atleast about 35% by weight polyvinylidene chloride (PVDC). As known tothose familiar with such materials, polyvinylidene chloride is thepolymer formed from vinylidene chloride or the copolymerization ofvinylidene chloride with lesser amounts of other unsaturated compounds.Vinylidene chloride is a colorless liquid having the formula C₂ H_(c)Cl₂ in which both hydrogen atoms are replaced by chlorine atoms on oneof the carbon atoms in ethylene. The copolymer included with thepolyvinylidene chloride, if one is used, can be selected from the groupconsisting of acrylic polymers, methacrylic polymers, nitryl polymers,itaconic acid (methylene succinic acid) polymers,

polyisocyanates, and polyisothiocyanates. In general, thePVDC-containing layer preferably has a thickness of about 1 micron (μ),in other words sufficient to accept the colloid layer 13 and thepolyvinylidene chloride layer 14, without any unnecessary excess, andwithout interfering with the optical or physical properties of the lens.

The water permeable colloid layer 13 preferably comprises gelatin. Asgenerally used in the art, the term "gelatin" refers to a mixture ofproteins obtained by hydrolysis of collagen by boiling skin, ligaments,tendons or the like. Type A gelatin is obtained from acid treated rawmaterials and Type B from alkali treated raw materials. Gelatin isstrongly hydrophilic, absorbing up to ten times its weight of water andforming reversible gels of high strength and viscosity. In preferredembodiments of the invention, the gelatin layer has a thickness of about1μ. As with the other layers, the thickness can be functionallydescribed as being sufficient to provide the bonding function while thinenough to avoid interfering with necessary properties such asflexibility, transparency, and other optical performance parameters.Thicknesses greater than those functionally appropriate are merely anexcessive and unnecessary use of the colloid.

The polyvinyl alcohol-containing layer 14 generally further comprisespolyvinyl acetate. The presence of polyvinyl acetate along withpolyvinyl alcohol is well known to those of ordinary skill in this artand results from the manner in which polyvinyl alcohol is manufactured;i.e. the hydrolysis of polyvinyl acetate to form alcohol functionalgroups. Stated differently, vinyl alcohol does not occur as a monomer,so that polyvinyl alcohol is generally formed by the hydrolysis ofpolyvinyl acetate. To the extent that hydrolysis is less than 100% (asis usually the case), the remainder of the polymer is polyvinyl acetate.In preferred embodiments, the polyvinyl alcohol-containing layer has anaverage molecular weight of between about 11,000 and 110,000 grams permole and is between about 80% and 99% hydrolyzed. In the most preferredembodiments, the polyvinyl alcohol-containing layer is at least about87% hydrolyzed.

In the preferred embodiments, the polyvinyl alcohol-containing layer hasa thickness of between about 0.1 and 0.5 mils (2.5 and 12.7μ). Ingeneral, the polyvinyl alcohol-containing layer can be any appropriatethickness to provide the fog-resistant characteristics, but generallyspeaking more than about 0.5 mil is excess and offers little or noadditional functional value.

In another embodiment, the invention comprises a method of preparing anantifogging plastic material that is particularly suitable for use adisposable fog-resistant lens. In one aspect, the method comprisesapplying the layer of polyvinyl alcohol-containing polymer to thesurface of a polyester substrate that has a polyvinylidenechloride-containing layer on the polyester substrate and a waterpermeable colloid layer on the polyvinyl chloride-containing layer. Inpreferred embodiments, the polyvinyl alcohol-containing polymer isapplied from an aqueous solution of polyvinyl alcohol.

As stated above, polyvinyl alcohol is generally formed from polyvinylacetate, thus the method of the invention can further comprise the stepsof polymerizing vinyl acetate and thereafter hydrolyzing the resultingpolyvinyl acetate to form the polyvinyl alcohol, both prior to the stepof applying the polyvinyl alcohol-containing polymer to thepolyvinylidene chloride-containing layer on the surface of the polyestersubstrate. As noted above, when the polyvinyl acetate is so hydrolyzed,the preferred technique is to hydrolyze the polyvinyl acetate to betweenabout 80% and 99% polyvinyl alcohol.

The polyvinyl alcohol-containing polymer can be applied in anyappropriate method. Typical methods include roll coating, rotogravurecoating, knife over roll coating, and Meyer rod coating. Presently, slotdie application is preferred. In such techniques, the polyvinylalcohol-containing polymer is applied from an aqueous solution at a rateof about 2 pounds per ream of substrate, which corresponds to athickness of about 0.1 mil (0.0001 inch). The polyvinylalcohol-containing solution can have a solids content of between about1% and 20%, and best results appear to be obtained when the solidscontent of the aqueous solution of polyvinyl alcohol is between about 5%and 10%. Preferably, the aqueous solution also contains a small fraction(e.g. 10%) of a low aliphatic alcohol such as ethyl, methyl, n-propyl,or isopropyl alcohol. The alcohol helps wet out the substrate andprevents fungus from growing in the wet media prior to its applicationand drying.

Taken on a stepwise basis, the invention further comprises the method ofapplying a layer of polyvinylidene chloride (PVDC) to the polyestersubstrate, then applying the layer of water permeable colloid to thePVDC-containing layer, and finally applying a layer of polyvinyl alcoholto the water permeable colloid layer on the PVDC layer.

Specific techniques for preparing polyester films with PVDC and colloidlayers thereon are known in the art and are exemplified by U.S. Pat. No.2,779,684 to Alles, the contents of which are incorporated entirelyherein by reference. Additionally, polyester substrates coated withvinylidene chloride are available under the CRONAR trademark from DuPontde Nemours E.I. Co., Wilmington, Del. 19898. In particular, the mostsatisfactory version of CRONAR found to date is available under theCRONAR-P designation.

EXAMPLE

One hundred fifty grams (150 g) of tap water were placed in a 250milliliter (ml) beaker and heated to 170° F. Fifteen and one tenth grams(15.1 g) of AIRVOL 203 polyvinyl alcohol (from Air Products andChemicals, 7201 Hamilton Blvd., Allentown, Pa. 18195) were added withcontinuous stirring until completely dissolved (about 30 minutes). Thesolution was allowed to cool to 90° F., after which 21.6 g of isopropylalcohol were added. This solution was filtered to remove anycontamination. The solution was then applied to 0.007 inch thick CRONARP72 polyester film (from DuPont) by the drawdown method using a #16Meyer Rod. The coated substrate was then dried in an air oven at 200° F.for two minutes to remove the solvents.

The resulting product exhibited excellent optical clarity, with a hazevalue of 0.2. In order to confirm its antifog properties, the productwas placed coated side down over a beaker containing 100 g of 150° F.water for a period of at least one minute, and did not allow water tocloud or fog its surface.

The adhesion of the coating to the substrate was tested using a ScotchBrand #600 adhesive tape in the Cross hatch tape test described in ASTMB 449. Using this test, the coating could not be removed from thesubstrate.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms havebeen employed, they have been used in a generic and descriptive senseonly and not for purposes of limitation, the scope of the inventionbeing set forth in the following claims.

That which is claimed is:
 1. An antifogging plastic lens materialcomprising:a substantially transparent polyester substrate; asubstantially transparent layer containing polyvinylidene chloride onsaid polyester substrate; a substantially transparent water permeablecolloid layer on said polyvinylidene chloride-containing layer; and asubstantially transparent layer containing polyvinyl alcohol on saidwater permeable colloid layer.
 2. An antifogging lens material accordingto claim 1 wherein said polyvinyl alcohol-containing layer furthercomprises polyvinyl acetate.
 3. An antifogging lens material accordingto claim 1 wherein said water permeable colloid layer comprises gelatin.4. An antifogging lens material according to claim 1 wherein saidpolyvinylidene chloride-containing layer contains at least about 35percent by weight polyvinylidene chloride.
 5. An antifogging lensmaterial according to claim 1 wherein said polyvinylidenechloride-containing layer includes a copolymer selected from the groupconsisting of: acrylic polymers, methacrylic polymers, nitrile polymers,itaconic acid polymers, polyisocyanates, and polyisothiocyanates.
 6. Anantifogging lens material according to claim 1 wherein said polyvinylalcohol-containing layer has an average molecular weight of betweenabout 11,000 and 110,000 grams per mole and is between about 80 and 99percent hydrolyzed.
 7. An antifogging lens material according to claim 1wherein said polyvinyl alcohol-containing layer is at least 87 percenthydrolyzed.
 8. An antifogging lens material according to claim 1 whereinsaid polyvinyl alcohol-containing layer has a thickness of between about2.5 and 12.7 microns.
 9. An antifogging lens material according to claim1 wherein said polyvinylidene chloride-containing layer has a thicknessof about 1 micron.
 10. An antifogging lens material according to claim 1wherein said water permeable colloid layer has a thickness of about 1micron.
 11. An antifogging lens material according to claim 1 whereinsaid polyester substrate is between about 101.6 and 177.8 microns thick.12. An antifogging lens material according to claim 1 in which saidpolyvinyl alcohol-containing layer cannot be removed from said substrateusing a Scotch Brand #600 adhesive tape in a Cross hatch tape testdescribed in ASTM B
 449. 13. A method of preparing an antifoggingplastic material that is particularly suitable for use as a disposablefog-resistant lens, said method comprising applying a layer of apolyvinyl alcohol-containing polymer to the surface of a polyestersubstrate that has a polyvinylidene chloride-containing layer on thepolyester substrate, and a water permeable colloid layer on thepolyvinylidene chloride-containing layer.
 14. A method according toclaim 13 wherein the step of applying the polyvinyl alcohol-containingpolymer comprises applying an aqueous solution of polyvinyl alcohol. 15.A method according to claim 14 wherein the step of applying thepolyvinyl alcohol from an aqueous solution comprises applying thepolyvinyl alcohol from an aqueous solution that contains about tenpercent of a low aliphatic alcohol.
 16. A method according to claim 13in which the step of applying the polyvinyl alcohol-containing polymercomprises applying the polymer from an aqueous solution at a rate ofabout 2 pounds per ream of substrate.
 17. A method according to claim 13in which the step of applying the polyvinyl alcohol-containing polymercomprises applying the polymer in a thickness of about 0.1 mil.
 18. Amethod according to claim 13 in which the step of applying the polyvinylalcohol-containing polymer comprises applying the polymer from anaqueous solution containing a solids content of between about 1 and 20percent.
 19. A method according to claim 13 in which the step ofapplying the polyvinyl alcohol-containing polymer comprises applying thepolymer from an aqueous solution containing a solids content of betweenabout 5 and 10 percent.
 20. A method of preparing an antifogging plasticmaterial that is particularly suitable for use as a disposablefog-resistant lens, said method comprising:applying a layer of apolyvinylidene chloride-containing polymer to a polyester substrate;applying a layer of a water permeable colloid to the polyvinylidenechloride-containing layer; and applying a layer of polyvinylalcohol-containing polymer to the water permeable colloid layer on thepolyviylidene chloride-containing layer.
 21. A method according to claim20 wherein the step of applying the polyvinyl alcohol-containing polymercomprises applying an aqueous solution of polyvinyl alcohol.